It’s popular because supernatural. In the show it’s a virus made by demons to wipe out entire cities, and there’s an episode where Dean goes into the future, and there’s a quarantine sign dated August 1 2014, hence the fandom freakout. But it’s based off folklore about the first colony at Roanoke, where a crew left the colony and when they returned it was completely abandoned, with only the word “CROATOAN” carved into a tree. Some researchers apparently think it was left behind by the settlers as a guidepost, as there was a Native American group near the Roanoke colony that was either called or had an area they called Croatoa (I can’t remember which). Anyone who remembers the actual history, please chime in and tell me where I’ve butchered it. I apologize in advance.